1. Field of the Invention
This invention relates to liquid crystal twist cells.
2. Description of the Prior Art
Liquid crystal devices have been the subject of intensive technological development for a number of years. Such devices may be utilized in optical memory units, display devices, or switches and require only small amounts of power for efficient operation. The liquid crystal devices fall into three distinct categories, depending upon their specific mode of operation,
In the "dynamic scattering devices" the transmission characteristics of the liquid crystal are altered by causing an ion current to flow through the crystalline material. The turbulence which is generated by this current flow is sufficient to render the otherwise transparent liquid opaque.
In a second class of devices the liquid crystal is mixed with an appropriate dye which is characterized by an anisotropic absorption coefficient. In this configuration the liquid crystal is used as a host which may be aligned by means of an electric field. The alignment of the liquid crystal host alters the alignment characteristics of the anisotropically absorbing dye. This alteration in the alignment of the dye affects its absorbing characteristics and hence alters the transmission properties of the device.
A third type of liquid crystal device--the twist cell-was first described by Schadt and Helfrich in Volume 18 of the Applied Physics Letters at page 127. Unlike the prior devices, the twist cell operates by rotating the direction of polarization of an incoming electro-magnetic wave.
In the twist cell a liquid crystal is sandwiched between two plane surfaces, with the liquid crystal molecules oriented approximately parallel to the surfaces. The molecules, although arranged in such a planar configuration, are still free to point in any given direction in the plane parallel to the surface. In a twist cell the distribution of directions of the individual molecules is made to continuously change from one surface to the other. In this manner a configuration of molecular orientations is formed which may be visualized as similar to the steps of a ladder which has been twisted into a helix about an axis which runs through the center of the steps. In this heuristic picture the steps represent the individual molecules. In a Schadt and Helfrich twist cell the total molecular rotation from one surface to another is about 90 degrees, and hence these devices are sometimes referred to as "quarter-turn twist cells."
According to a well-known optical principle, light entering the crystal, polarized in the plane of the liquid crystal molecules near the entering surface, and either parallel or perpendicular to the molecules, has its direction of polarization rotated as it traverses the crystal. The amount of rotation is equivalent to the amount of molecular twist.
If an electric field is applied to the crystal and is caused to point from one surface to the other, then the molecules in the central region of the cell will be aligned approximately parallel to the electric field rather than approximately parallel to the liquid crystal boundary surfaces. Such molecules will be approximately perpendicular to their previous "field off" position. In such a configuration light entering the crystal as before will not have its direction of polarization rotated and hence will traverse the liquid crystal unaffected.
It is apparent that if a polarizer is placed at the exit of a twist cell and is aligned parallel to the cell surface, but with its polarizing direction perpendicular to the alignment of the molecules closest to the exit surfaces, the light will only be transmitted through the polarizer when the field is on. This may be understood by considering that when the field is off, the polarizing direction of the polarizer is aligned to accept light whose initial polarization direction would not permit passage through the polarizer, but whose rotation by the twist cell permits such passage. However, when the field is on, the direction of polarization of the incoming light is not altered as it traverses the cell. Consequently, the light exits the crystal polarized in the same direction as when it entered the cell. Since the polarizing direction of the polarizer is aligned in this direction, the light will be transmitted through this polarizer. A reflecting surface placed beyond the polarizer will cause the light to be reflected back through the cell for viewing purposes. If the polarizer is rotated by 90 degrees the cell will be "off" when the field is on, as opposed to the above mode of operation.
While the above device is exceedingly attractive for use as an optical switch, a memory unit, or a display device, two phenomena limit its application.
Firstly, it is found that when the field is turned off, the transmission characteristics of the cell do not change monotonically but rather display an oscillatory behavior known as "optical bounce." A certain amount of this effect is due to a nonuniform relaxation phenomenon known as "backflow" whose effect may be reduced by the addition of some cholesteric, or naturally twisted liquid crystal.
A second limiting effect appears when the impinging light is not viewed from a perpendicular direction. In such a case, the birefringence of the liquid crystal material gives rise to an interference phenomenon. This results in a display with significant nonuniformity in intensity depending on the direction from which the display is viewed, and in an optical flicker or bounce when the device is turned on or off.